Although terrestrial wind engines have been built over the last few centuries, building wind turbines at sea is much more recent.
A modern wind turbine, whether for use on land or off shore, generally comprises a horizontal axis engine having a plurality of blades and an electricity generator coupled to the engine, both of them being fastened to the top end of a vertically elongate support such as a mast or pylon.
In order to reduce the cost of wind-generated energy and increase the efficiency of generators, ever more powerful generators are being fabricated that are installed in grouped manner so as to form a wind-turbine farm or field.
Increasing the power of a wind-turbine generator involves also increasing its weight and the height of the structure that supports it.
For economic reasons, it is necessary to implement wind turbines that are ever more powerful and thus of ever greater dimensions, and in particular of ever greater heights. Similarly, such wind turbines are being installed at ever greater distances from the shore and therefore in ever greater depths.
The invention applies particularly, i.e. in non-limiting manner, to wind turbines having a generator of power lying in the range 100 kilowatts (kW) to 10 megawatts (MW), the weight of such a generator possibly reaching or exceeding 300 (metric) tonnes (t) or 500 t; the length of a pylon supporting the generator may be of the order of 50 meters (m) to 100 m, and the weight of the pylon may lie in the range 100 t to 500 t.
It can thus be understood that constructing such wind turbines and installing them at sea presents great difficulties. Various solutions have been proposed for constructing and installing off-shore wind turbines.
The wind turbine is generally constructed using conventional traction hoist means such as crane type hoists. An assembly is built comprising a base surmounted by a pylon having a wind engine and an electricity generator fastened to the top thereof.
Nevertheless, once they have been constructed, wind turbines present dimensions that are large and constitute loads that are heavy, as mentioned above, such that such wind turbines are not easy to transport at sea to their operating sites.
Furthermore, it can be understood that installing a wind turbine using cranes acting by traction requires the cranes to be of large dimensions, and in particular of height that is greater than the height of the wind turbine. For wind turbines of the above-mentioned dimensions, it is necessary to use cranes weighing at least 350 t and having a jib having a length of at least 50 m and generally closer to 100 m, where such cranes are difficult to mount and to move, and in particular to transport and to operate at sea. It is not reasonable to envisage transporting such crane-type hoist means on floating barges, as is sometimes appropriate for certain operations in off-shore oil fields, given the size they represent and the instability of a barge in a rough sea, with this applying both during transport and during hoisting operations.
Off-shore wind turbines are described in WO 01/34977 and WO 03/004870, for example. They are assembled on land and then transported by boat to their operating sites at sea away from the shore. In WO 01/34977 the wind turbine includes watertight tanks that can be ballasted with sea water or de-ballasted during transport. More particularly, the ship includes a narrow slot of width that enables only the pylon to be inserted therein, the base of the wind turbine being below the hull of the ship, and ballasted in part so that, by virtue of its buoyancy, its top face exerts thrust against the bottom face of the hull of the ship. However, during transport, fastening of the wind turbine to the ship is not reliable and requires not only very calm sea conditions but also subjects the fastening between the pylon and the base to high levels of stress that may harm the mechanical reliability of said fastening. Furthermore, the effects of swell and current on the base during transport give rise to forces that are considerable and thus to bending moments that are considerable at the device secured to the ship for gripping the pylon, thereby requiring large amounts of localized reinforcement in the grip system. Finally, the wind turbine is not guided in stabilized manner while it is being ballasted in order to be placed on the sea bottom.
WO 03/066426 describes a catamaran boat having two side floats constituting a U-shaped floating structure suitable for transporting a wind turbine at sea while it is placed between the two side floats, and for placing it on the sea bottom. The boat is fitted with devices for hoisting/lowering said wind turbine, said hoisting/lowering device and/or said wind turbine moving vertically relative to the two side floats in order to move the wind turbine from the surface to the bottom of the sea. That device for hoisting/lowering the wind turbine and the means for holding and guiding said wind turbine between the two side floats represent bulk and weight that are large on the deck of the U-shaped floating structure.